The use of SRGs is becoming more widespread with the availability of high-speed sensing and control systems for controlling their operation in varying operating conditions. These uses include aerospace power systems, i.e., driven by a turbine at very high sustained speeds, wind turbines, i.e., where blade geometry in conjunction with wind speed maintain predictable high torque and low speed inputs, and in hybrid drive automotive systems, where the switched reluctance machine (SRM) operates as both a motor or generator as necessary.
The control of an SRM is based on three variables. The reference current I, the turn-on angle θon and the turn-off angle θoff. The SRM produces torque, i.e., a reaction to spinning the rotor with respect to the stator, either positively (for motor function) or negatively (for generator function) depending on what the controller requires of the SRM. As a motor, the SRM control is relatively uncomplicated. A current is made available, and a turn-on angle is selected depending on a desired torque characteristic of the motor, that is, how much twist is required from the SRM at that moment. The turn-off angle is determined by choosing an angular duration of a torque pulse, i.e., the conduction angle (the angular difference between the turn-on and turn-off angles). Essentially, the operator chooses how much and how long a “pull” is placed on each rotor element as it approaches and passes each stator element.
Operating the SRM in a net “negative” torque mode as a generator is rather more complicated. The same parameters of control are available. That is, the turn-on and turn-off angles, but in this instance the current is usually a target inasmuch as it is being supplied to an electrical load as a result of electrical generation demand in the system. Hence, to have straight forward control of the SRM as a generator it is useful to apply a known input torque at a known rate so that depending on the output current and power desired, the turn-on and turn-off angles and resultant conduction angle can be chosen and the current and power demands reliably achieved.
However, in the application where an SRM is driven in an automotive IC engine setting, it is almost never the case that a known input torque and rate are supplied. Instead these inputs are transient as an IC engine transitions from idle to acceleration to constant speed to regenerative braking effect back to wide open throttle, etc., in the ordinary course of use. In such a circumstance where the rate and amount of input torque can vary widely, the combination of turn-on and turn-off angles that produce a specified output current from the SRM operating as an SRG are essentially endless. As such, control of an SRG in such operating conditions is quite difficult.
In addition, when the SRM is operated as a motor, it is the turn-on angle only that determines peak phase current. When the SRM is operated as a generator, both turn-on angle and turn-off angle influence the peak phase current. There are multiple combinations of turn-on and turn-off angles that are able to produce the same amount of average torque. This raises the issue of how to best select the turn-on and turn-off angle in order to achieve the required torque subject to some other control objective, such as maximizing efficiency.
Further, in selecting an SRM for automotive applications efficiency is paramount. The desire is to provide the necessary motive power, i.e., as a starter for the IC engine, while not adding extra weight to be carried by the vehicle in question and, at the same time, providing sufficient generating capacity for the electric power demands existing in the system. The ratio of mechanical input versus electrical output should be optimized for efficient operation.